The invention relates generally to an apparatus for treating particulate materials.
More particularly, the invention relates to an apparatus for crystallizing particulate materials.
A more specific aspect of the invention deals with a apparatus for crystallizing granular synthetic resins, especially granular polyesters.
Methods for the crystallization of granular synthetic resins operate primarily with so-called fluidizing heaters or fluidizing dryers. These heaters or dryers are pot-shaped vessels equipped with special inserts or stirrers. The granular resin is admitted into such a vessel in discontinuous batches and is subjected to an upwardly directed stream of hot gas. The gas stream forms a fluidized bed of the granular resin which is held in a freely suspended state while simultaneously, being subjected to a whirling motion, for a period of time. As a result of this treatment, which involves heating the granular resin to its transition temperature, the resin transforms from the amorphous or non-crystalline state to the crystalline state. The whirling motion imparted to the resin particles with the aid of the inserts and the stream of hot gas prevents the particles from adhering to one another.
The known apparatus of this type such as disclosed, for example, in the French Pat. No. 1,352,614 and the German Pat. No. 2,155,785, exhibit certain disadvantages. They use large amounts of energy and, as a result of this and the costly inserts provided therein, require large expenditures. A further disadvantage resides in that the crystallization process is discontinuous.
Continuous crystallization processes are known from French Pat. No. 1,478,224 and German Offenlegungsschrift No. 1,779,521. One of these processes operates with a fluidizing arrangement whereas the other operates by passing the material through the crystallization zone in compact bulk form under the influence of gravity and subjecting the material to stirring. However, these processes, as well as the corresponding apparatus, exhibit the common disadvantage of being suitable only for granulated products such as, for example, the so-called granular band or strand materials, which have relatively little tendency to form clumps or aggregates by adhesion of the particles to one another. The reason that granular materials of this type, which in the main consist of particles of parallelepiped form, have little tendency to form clumps stems from the fact that the manufacturing process for these granular materials causes the rate of primary crystallization at the sectional planes and the outer surfaces of the particles to be different. Thus, a tendency for the particles to bake together, or to adhere to one another, exists only for a single orientation and thereby reduces the tendency for clump formation. Furthermore, even this reduced tendency for clump formation may be limited by suitable selection of the temperature or may be eliminated by the use of stirrers.
German Offenlegungsschrift No. 1,779,521 discloses mechanical means in the form of rollers provided with pins for breaking up clumps of particles. However, this type of mechanical means was intended for use only with the type of granular material just described, namely, a granular material having little tendency to form clumps. In fact, mechanical means such as described in the German Offenlegungsschrift No. 1,779,521 have heretofore been used only under abnormal conditions, e.g. incorrect temperatures, breaking of stirring paddles, etc., and consequently have found utilization only in exceptional cases for breaking up small blocks or clumps of granular band or strand materials.
These latter types of granular materials are gradually being replaced by so-called underwater granular materials. These are granular materials which are produced by making strands of cylindrical cross-section from a melt and then cutting the strands underwater while still in the plastic state. Products of the type which are granulated underwater are much preferred inasmuch as energy consumption and surface area requirements are substantially smaller than are those for granular band or strand materials. However, by virtue of the shape of the particles of this underwater granular material, as well as by virtue of a very low and a very uniform rate of primary crystallization, there exists an extremely pronounced binding tendency in all orientations. In other words, an extensive baking together or adhesion of the particles of the granular material occurs in all directions upon heating. As a result, all of the known crystallization processes which operate with stirring or fluidizing arrangements become completely ineffective after only a short period of time.